Extended thermal stage sulfur recovery process

ABSTRACT

A process for recovering sulfur in a sulfur recovery unit comprising the steps of reacting hydrogen sulfide and oxygen in the reaction furnace at a minimum reaction temperature to produce a reaction effluent; reducing the temperature of the reaction effluent from the minimum reaction temperature to a boiler section outlet temperature to produce a cooled effluent, the cooled effluent comprises hydrogen sulfide, sulfur dioxide, and sulfur-containing contaminants; reacting the hydrogen sulfide, sulfur dioxide, and sulfur-containing contaminants in the catalytic extension to produce a boiler catalytic effluent; reducing the boiler catalytic effluent temperature such that the elemental sulfur condenses to form liquid sulfur and a gases stream; reacting the hydrogen sulfide and sulfur-containing contaminants with the oxygen to produce an oxidizer outlet stream comprises sulfur dioxide; and separating the sulfur dioxide in the scrubbing unit to produce a recycle stream and an effluent gases, the recycle stream comprises sulfur dioxide.

BACKGROUND OF THE ART Technical Field

Disclosed are an apparatus and process for recovery of elemental sulfur.More specifically, disclosed are an apparatus and process for recoveryof elemental sulfur and removal of sulfur-containing contaminants froman acid gas stream.

Background

The sulfur recovery industry has been using the Claus gas phasereactions as the basis for recovering elemental sulfur from hydrogensulfide (H₂S) since the 1940s. The Claus plant, the long-standing‘workhorse’ of the industry, uses the Claus chemistry to achieve between96% and 98% recovery of elemental sulfur from the hydrogen sulfide in anacid gas streams. The gas phase elemental sulfur from the Claus plant issubsequently condensed and collected in the liquid form.

The vast majority of all operating Claus plants worldwide include athermal stage (including a free-flame reaction furnace and a wasteheatboiler) followed by either 2 or 3 catalytic stages, with each stageincluding a reheater, a catalytic converter, and a condenser. The Clausplants result in recovery efficiencies of 96 percent (%) for a 2-stagedesign or 98% for a 3-stage design. There are only a handful of 4-stagedesigns in the world because, early on, the sulfur recovery industryrecognized that a 4^(th) catalytic stage only marginally increased therecovery efficiency greater than 98% and was therefore uneconomical.

Owing to the negative impact of acid rain, formed due to high levels ofsulfur dioxide (SO₂) in the atmosphere, local environmental regulatorybodies imposed limits on the amount of sulfur dioxide emitted in theeffluent of Claus plants. In response, the industry began developingTail Gas Treatment (TGT) technologies to be placed immediatelydownstream of the Claus plant to further improve the recovery efficiencyof the sulfur recovery unit to greater than 99%, or in some casesgreater than 99.9%, while removing SO₂ from the effluent.

While the Claus plant does provide a path for recovery of sulfur, it isnot without drawbacks. The catalytic stages require regeneration andreplacement due to catalyst fouling plugging and various deactivationmechanisms. Replacement of catalyst can require significant downtime,potentially putting the entire processing unit offline. The catalyticstages are sensitive to the presence of contaminants and to thetemperature of the catalytic feed stream. These sensitivities can makecontrolling the catalytic stages cumbersome.

SUMMARY

Disclosed are an apparatus and process for recovery of elemental sulfur.More specifically, disclosed are an apparatus and process for recoveryof elemental sulfur and removal of sulfur-containing contaminants froman acid gas stream.

In a first aspect, a process for recovering sulfur in a sulfur recoveryunit is provided. The process includes the steps of introducing an acidgas feed to a reaction furnace, where the acid gas feed includeshydrogen sulfide, introducing a fuel gas to the reaction furnace,introducing air feed to the reaction furnace, where the air feedincludes oxygen, reacting the hydrogen sulfide and oxygen in thereaction furnace at a minimum reaction temperature to produce a reactioneffluent, where the reaction effluent includes elemental sulfur andsulfur dioxide, where the reaction effluent is at the minimum reactiontemperature, introducing the reaction effluent to a wasteheat stage ofan extended boiler stage, reducing the temperature of the reactioneffluent from the minimum reaction temperature to a boiler sectionoutlet temperature to produce a cooled effluent that includes hydrogensulfide, sulfur dioxide, and sulfur-containing contaminants, where thetemperature of the reaction effluent is reduced by capturing heatenergy, introducing the cooled effluent to a catalytic extension of theextended boiler stage, where the catalytic extension includes a catalystand has a gross hourly space velocity between 3000 h⁻¹ and 6000 h⁻¹,reacting the hydrogen sulfide, sulfur dioxide, and sulfur-containingcontaminants in the catalytic extension to produce a boiler catalyticeffluent that includes elemental sulfur, where the boiler catalyticeffluent is at a boiler catalytic effluent temperature, introducing theboiler catalytic effluent to a condenser, where the boiler catalyticeffluent includes elemental sulfur, reducing the boiler catalyticeffluent temperature such that the elemental sulfur condenses to formliquid sulfur and a gases stream, introducing the gases stream to athermal oxidizer, where the gases stream includes hydrogen sulfide andsulfur-containing contaminants, introducing an air stream to the thermaloxidizer, where the air stream includes oxygen, reacting the hydrogensulfide and sulfur-containing contaminants with the oxygen to produce anoxidizer outlet stream that includes sulfur dioxide, introducing theoxidizer outlet stream to a scrubbing unit, and separating the sulfurdioxide in the scrubbing unit to produce a recycle stream and aneffluent gases, where the recycle stream includes sulfur dioxide.

In certain aspects, the process further includes the step of introducingthe recycle stream to the reaction furnace. In certain aspects, theprocess further includes the steps of introducing a boiler feed water tothe extended boiler stage, and increasing the temperature of the boilerfeed water to produce a high pressure steam, where the heat capturedfrom the reaction effluent is used to increase the temperature of theboiler feed water. In certain aspects, the minimum reaction temperatureis between 850 deg C. and 1250 deg C. In certain aspects, the boilersection outlet temperature is between 148 deg C. and 254 deg C. Incertain aspects, the boiler catalytic effluent temperature is between250 deg C. and 400 deg C. In certain aspects, the catalyst is titaniaextrudate. In certain aspects, an overall conversion of sulfur compoundsto elemental sulfur is greater than 99.9 mol %. In certain aspects, theprocess further includes the steps of analyzing a composition of thegases stream in a tail gas analyzer; and adjusting a flow rate of theair feed based on the composition of the gases stream to maintain astoichiometric ratio of hydrogen sulfide to sulfur dioxide of 2:1. Incertain aspects, the process further includes the steps of measuring theminimum reaction temperature in the reaction furnace with a temperaturesensor and adjusting a flow rate of the fuel gas to maintain the minimumreaction temperature.

In a second aspect, an apparatus for recovering sulfur from an acid gasstream is provided. The apparatus includes a reaction furnace configuredto contain a reaction between hydrogen sulfide in the acid gas streamand oxygen in an air feed to produce a reaction effluent, where thereaction furnace operates at a minimum reaction temperature, where thereaction effluent includes elemental sulfur; an extended boiler stagethat includes a wasteheat stage fluidly connected to the reactionfurnace, the wasteheat stage configured to capture heat energy from areaction effluent to produce a cooled effluent at a boiler sectionoutlet temperature, and a catalytic extension physically connected tothe wasteheat stage, the catalytic extension configured to allow areaction to convert sulfur compounds to produce a boiler catalyticeffluent that includes elemental sulfur, where the catalytic extensionincludes a catalyst section that includes a catalyst; a condenserfluidly connected to the catalytic extension, the condenser configuredto condense the elemental sulfur to produce liquid sulfur and a gasesstream; a thermal oxidizer fluidly connected to the condenser, thethermal oxidizer configured to oxidize sulfur compounds in the gasesstream and oxygen in an air stream to produce an oxidizer outlet streamthat includes sulfur dioxide; and a scrubbing unit fluidly connected tothe thermal oxidizer, and the scrubbing unit configured to separate thesulfur dioxide from the gases stream to produce a recycle stream and aneffluent gases stream, where the recycle stream includes the sulfurdioxide.

In certain aspects, the wasteheat stage includes a heat exchanger. Incertain aspects, the catalytic extension has a gross hourly spacevelocity between 3000 h−1 and 6000 h−1. In certain aspects, theapparatus further includes a tail gas analyzer fluidly connected to thecondenser, the tail gas analyzer configured to analyze a composition ofthe gases stream. In certain aspects, the apparatus further includes atemperature sensor fluidly connected to the reaction furnace, thetemperature sensor configured to measure the minimum reactiontemperature in the reaction furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages will become betterunderstood with regard to the following descriptions, claims, andaccompanying drawings. It is to be noted, however, that the drawingsillustrate only several embodiments and are therefore not to beconsidered limiting of the scope as it can admit to other equallyeffective embodiments.

FIG. 1 is a graphical representation showing the Gamson & Elkinsconversion relationship.

FIG. 2 is a process diagram of an embodiment.

FIG. 3 is a process diagram of an embodiment.

FIG. 4 is a process diagram of an embodiment.

FIG. 5 is a process diagram of an embodiment.

DETAILED DESCRIPTION

While the scope of the apparatus and method will be described withseveral embodiments, it is understood that one of ordinary skill in therelevant art will appreciate that many examples, variations andalterations to the apparatus and methods described here are within thescope and spirit of the embodiments. Accordingly, the exemplaryembodiments described here are set forth without any loss of generality,and without imposing limitations.

The apparatus and method described here are directed to a sulfurrecovery unit that includes an extended boiler stage following thereaction furnace and do not include a conventional Claus catalyticstage. The extended boiler stage includes a wasteheat stage to removeheat from the reaction effluent and a catalytic extension to catalyzeClaus reactions and reactions to convert sulfur-containing contaminants.

Advantageously, the use of the catalytic extension following thewasteheat stage takes advantage of the Gamson & Elkins conversionrelationship and allows the Claus reaction to proceed further due to theremoval of heat from the system, without the need for product removal.The Gamson & Elkins conversion relationship can be described withrespect to FIG. 1. To understand the Gamson & Elkins conversionrelationship, begin at a point on the graph at 2000 degrees Fahrenheit(deg F.), the conversion (shown along the y-axis) is approximately 70percent. Reducing the temperature of the gas mixture to 600 deg F., theconversion is approximately 85 percent, without removing any of theClaus reaction products, such as elemental sulfur or water. Thus, theextended boiler stage of the sulfur recovery unit takes advantage ofthis Gamson & Elkins conversion relationship which shows that the Clausreaction equilibrium can be shifted to the right without the need toremove products, as is normally the case to shift equilibrium reactions.In a conventional Claus process, the elemental sulfur reaction productis removed in condensers in order that the equilibrium favors theproduction of more elemental sulfur product in the next catalyticreactor.

Advantageously, the sulfur recovery unit can recover sulfur from a leanacid gas stream without accumulation of sulfur compounds in the system,while allowing for a full recycle of sulfur dioxide recovered in thescrubbing unit.

Advantageously, the sulfur recovery unit results in reduced capitalexpenditures by 30 to 50% due to a reduction in the number of vesselsneeded. Advantageously, the sulfur recovery unit results in reducedlifecycle operating expenditures due to reduced costs attributable tocatalyst regeneration and replacement. Advantageously, the sulfurrecovery unit results in reduced plot plan size, resulting in savedphysical space. Advantageously, the sulfur recovery unit can result inoverall recovery efficiency of greater than 99.99%.

The sulfur recovery unit with an extended boiler stage has a oncethrough conversion of sulfur compounds to elemental sulfur of 85%, whichis less than the conventional Claus process which has a once throughconversion of sulfur compounds to elemental sulfur of typically 96% to98%. However, the sulfur recovery unit, as a whole, can obtain anoverall conversion of sulfur compounds to elemental sulfur of greaterthan 99.9%.

As used here, “conventional Claus process” refers to a process thatincludes a reaction furnace followed by two to three conventional Clauscatalytic stages.

As used here, “conventional Claus catalytic stage” refers to thecatalytic stage of a conventional Claus process that includes areheater, a catalytic converter, and a sulfur condenser.

As used here, “gross hourly space velocity” or “gas hourly spacevelocity” refers to a measure of the reactant gas flow rate divided bythe reactor volume measured at standard temperature and pressure. In thesulfur recovery unit it is a measure of the standard volumetric flowrate of the process gas to the extended boiler stage divided by thevolume of the catalyst in the extended boiler stage.

As used here, “lean acid gas” refers to an acid gas containing less than50% by volume hydrogen sulfide.

As used here, “medium acid gas” refers to an acid gas containing 50% byvolume to 75% by volume hydrogen sulfide.

As used here, “high acid gas” refers to an acid gas containing greaterthan 75% hydrogen.

As used here, “process gases” refers to gases that can include carbonmonoxide (CO), carbon dioxide (CO₂), water (H₂O), nitrogen (N₂),hydrogen (H₂), argon (Ar), and combinations of the same.

As used here, “process contaminants” refers to contaminants that caninclude hydrocarbons, benzene, toluene, ethylbenzene, and xylene (BTEX),methanol (CH₃OH), ammonia (NH₃), and combinations of the same.

As used here, “sulfur-containing contaminants” refers to contaminantsthat can include carbonyl sulfide (COS), carbon disulfide (CS₂),mercaptans, and combinations of the same.

As used here, “sulfur-conversion products” refers to reaction productsthat can include elemental sulfur, sulfur dioxide (SO₂),sulfur-containing contaminants, and combinations of the same.

As used here, “sulfur compound” refers to a compound that includessulfur.

As used here, “in the absence of” means does not contain, does notinclude, is not a part of, or is not a component of.

Referring to FIG. 2, a process diagram of an embodiment of sulfurrecovery unit 1 is provided.

Acid gas feed 100, fuel gas 102 and air feed 104 are introduced toreaction furnace 10. Acid gas feed 100 can be from any source of acidgas. Acid gas feed 100 can be a lean acid gas, a medium acid gas, or ahigh acid gas. Acid gas feed 100 can contain hydrogen sulfide, processgases, process contaminants, and sulfur-containing contaminants. Thenature and composition of the process gases, process contaminants, andsulfur-containing contaminants depends on the process that is the sourcefor acid gas feed 100. The precise composition of acid gas feed 100depends upon the source and can be determined using any technologycapable of analyzing the composition of an acid gas feed stream. In atleast one embodiment, the source of acid gas feed 100 is a refinery, andacid gas feed 100 includes ammonia. In an alternate embodiment, thesource of acid gas feed 100 is a sour gas plant and acid gas feed 100 isin the absence of ammonia. In at least one embodiment, there are nolimits to the amount of hydrogen sulfide that can be present in acid gasfeed 100.

Fuel gas 102 can be any fuel gas suitable for co-firing in reactionfurnace 10. Fuel gas 102 provides additional fuel to increase and/ormaintain the temperature in reaction furnace 10. In at least oneembodiment, fuel gas 102 is natural gas. In at least one embodiment,fuel gas 102 includes hydrocarbons having between one and six carbonatoms (C₁-C₆).

Air feed 104 can be any oxygen (O₂) containing gas suitable for use inreaction furnace 10. Exemplary gases suitable for use as air feed 104include air, oxygen enriched air, pure oxygen, or any combination of thesame. In at least one embodiment, air feed 104 is air. In at least oneembodiment, the volumetric flow rate of air feed 104 is provided suchthat oxygen is in stoichiometric excess relative to the fuel gas, suchthat all of the fuel gas can be burned and the temperature in thereaction furnace can be maintained. In at least one embodiment, thevolumetric flow rate of air feed 104 is provided such that oxygen is instoichiometric excess relative to the volume of fuel gas in fuel gas 102and excess oxygen is provided such that the ratio of hydrogen sulfide tosulfur dioxide at the outlet of the reaction furnace is about 2:1. Thevolumetric flow rate of air feed 104 can be controlled throughinstrumentation such as a tail gas analyzer.

Reaction furnace 10 is any reaction furnace suitable to combust hydrogensulfide and other components. Reaction furnace 10 can be designed andoperated to convert hydrogen sulfide and the sulfur-containingcontaminants to sulfur-conversion products. Reaction furnace 10 isdesigned and operated to destroy the process contaminants. In at leastone embodiment, reaction furnace 10 can be designed and operated tomaximize destruction of the sulfur-containing contaminants. In at leastone embodiment, reaction furnace 10 can be designed and operated tomaintain a ratio of hydrogen sulfide to sulfur dioxide at the reactionfurnace outlet of 2:1. Maximizing destruction of the sulfur-containingcontaminants increases the purity of sulfur recovered from sulfurrecovery unit 1 and eliminates the potential for sulfur-containingcontaminants to cause operating problems in units downstream of thereaction furnace. The temperature of reaction furnace 10 affects thesulfur-conversion products present in reaction effluent 110 and theamount of process contaminants that are destroyed. Reaction furnace 10heats the components of air feed 104, fuel gas 102, and acid gas feed100 to a minimum reaction temperature. The minimum reaction temperatureensures proper destruction of the process contaminants. The minimumreaction temperature is in the range of 850 degrees Celsius (deg C.) to1250 deg C. In at least one embodiment, acid gas feed 100 is in theabsence of ammonia and the minimum reaction temperature is at leastabout 1050 deg C. In at least one embodiment, acid gas feed 100 includesammonia and the minimum reaction temperature is 1250 deg C. In at leastone embodiment, reaction furnace 10 is in the absence of catalyst. In atleast one embodiment, sulfur recovery unit 1 can include preheating, ina heating unit, of acid gas feed 100. In at least one embodiment, sulfurrecovery unit 1 can include preheating, in a heating unit, of air feed104. The concentration of oxygen in reaction furnace 10 affects thetemperature. In at least one embodiment, sulfur recovery unit 1 caninclude preheating, in a heating unit, of fuel gas 102. The addition offuel gas 102 increases the temperature in reaction furnace 10.

The components in acid gas feed 100 react with oxygen in air feed 104.Conversion of hydrogen sulfide and the sulfur-containing contaminants toelemental sulfur (as vapor) and other sulfur-conversion products occursin reaction furnace 10. The conversion of hydrogen sulfide to elementalsulfur occurs according to the following reactions:2H₂S+3O₂→2SO₂+2H₂O  reaction 12H₂S+SO₂→3S+2H₂O  reaction 2

The conversion of hydrogen sulfide and the sulfur-containingcontaminants entering reaction furnace 10 to elemental sulfur is between55 mole percent (mol %) and 85 mol %, alternately between 60 mol % and80 mol %, alternately between 65 mol % and 75 mol %, alternately between68 mol % and 72 mol %. The conversion of hydrogen sulfide to elementalsulfur occurs in the absence of catalyst.

Destruction reactions of process contaminants can occur in reactionfurnace 10. The process contaminants present in acid gas feed 100 can bereduced by 95 percent by weight (wt %), alternately by 97 wt %,alternately by 99 wt %, alternately by 99.5 wt %, alternately by 99.9 wt%.

Reaction effluent 110 can contain hydrogen sulfide, sulfur-conversionproducts, process gases, process contaminants, and combinations of thesame. The exact composition of reaction effluent 110 depends on thecomposition of acid gas feed 100 and the conditions in reaction furnace10, including the minimum reaction temperature. The amount of hydrogensulfide, process gases, process contaminants, and sulfur-containingcontaminants present in reaction effluent 110 are reduced relative tothe amount of those components present in acid gas feed 100. Reactioneffluent 110 exits reaction furnace 10 at the minimum reactiontemperature and is introduced to extended boiler stage 20.

Referring to FIG. 3, a process flow diagram of an embodiment of extendedboiler stage 20 is provided. Extended boiler stage 20 includes wasteheatstage 22 and catalytic extension 24. Wasteheat stage 22 and catalyticextension 24 can be contained in one vessel. Wasteheat stage 22 can beany type of device capable of removing heat from a fluid to producesteam at desired process conditions. In at least one embodiment,wasteheat stage 22 is a single-pass shell and tube heat exchanger.Catalytic extension 24 can be physically attached to wasteheat stage 22.Catalytic extension 24 can be type of vessel or pipe capable of beingbolted on to wasteheat stage 22. Catalytic extension 24 can have adiameter and a length. In at least one embodiment, catalytic extension24 is the same diameter as wasteheat stage 22. Catalytic extension 24can have a length between 8 feet and 20 feet and alternately between 10feet and 15 feet. Catalytic extension 24 can be sized based on a grosshourly space velocity (GHSV) between 3000 per hour (h−1) and 6000 h−1. AGHSV between 3000 h−1 and 6000 h−1 means catalytic extension 24 is 3 to8 times smaller than a conventional Claus catalytic stage.Advantageously, the combination of the wasteheat stage and the catalyticextension can reduce the size of the catalytic extension as compared tothe catalytic converter in a conventional Claus catalytic stage, whichtypically has a GHSV of between 700 h−1 and 1000 h−1, while stillachieving equilibrium with respect to the Claus reaction in thecatalytic extension. Advantageously, the reduced size of the catalyticextension can reduce the economics and reduce the plot area necessary tosupport the extended boiler stage. Catalytic extension 24 can contain acatalyst section. The catalyst section can include a cage to hold thecatalyst in place. The catalyst section can contain a catalyst. Thecatalyst can be any catalyst capable of catalyzing the Claus reactionand hydrolyzing sulfur-containing contaminants. Examples of catalystinclude alumina spheres and titania extrudate. In at least oneembodiment, the catalyst can include titania, which can achieve anincreased rate of conversion of sulfur-containing contaminants ascompared to alumina. The increased rate of conversion ofsulfur-containing contaminants results in an increased overallconversion of the sulfur recovery unit, which reduces the sulfur dioxiderecycled to the reaction furnace. Advantageously, the catalyticextension hydrolyzes the carbonyl sulfide and carbon disulfide producedin the reaction furnace to hydrogen sulfide, which can then participatein the Claus reactions in the catalytic stage and increase the overallconversion. The production of carbonyl sulfide and carbon disulfide inthe reaction furnace negatively impacts the overall conversion ofsulfur, so the hydrolysis in the catalytic extension increases theoverall conversion. The addition of the catalytic extension increasesthe conversion of sulfur compounds resulting in reduced amounts ofsulfur dioxide being recycled to the reaction furnace than a system inthe absence of a conventional Claus catalytic stage. Reduced amounts ofsulfur dioxide being recycled can result in a reduction in the flow rateof fuel gas to the reaction furnace.

Reaction effluent 110 is introduced to wasteheat stage 22 of extendedboiler stage 20. Wasteheat stage 22 captures heat energy from reactioneffluent 110 to produce cooled effluent 124. The heat energy capturedfrom reaction effluent 110 can be used to heat boiler feed water 122 togenerate high pressure steam 125. Boiler feed water 122 can be anysource of water suitable for use to produce steam. High pressure steam125 can be saturated steam at a pressure greater than 50 pounds persquare inch gauge (psig) and alternately between 50 psig and 600 psig.Cooled effluent 124 is at a boiler section outlet temperature, wherewasteheat stage 22 reduces the temperature of reaction effluent 110 fromthe minimum reaction temperature to the boiler section outlettemperature. The amount of heat energy captured from reaction effluent110 can control the boiler section outlet temperature. The boilersection outlet temperature can be between 148 deg C. and 254 deg C.

Cooled effluent 124 flows from wasteheat stage 22 to catalytic extension24. Cooled effluent 124 can contain sulfur-conversion products, hydrogensulfide, process gases, process contaminants, and sulfur-containingcontaminants. Hydrogen sulfide, sulfur dioxide, and sulfur-containingcontaminants can be converted to elemental sulfur in catalytic extension24.

Catalytic extension 24 can increase the overall conversion of sulfur insulfur recovery unit 1 from 40 percent (%) to 75% in reaction furnace 10to 75% to 85% after extended boiler stage 20 in boiler catalyticeffluent 120. Advantageously, extended boiler stage 20 reduces theamount of sulfur compounds that will be recycled to the reaction furnaceand avoids a sulfur dioxide accumulation effect that can occur when theoverall conversion in sulfur recovery unit 1 is less than 66.6%. For alean acid gas, the conversion in reaction furnace 10 can be less than66%, typically between 40% and 65%; the addition of extended boilerstage 20 increases the conversion to greater than 66.6%.

The reactions in catalytic extension 24 can be exothermic, such thatboth the Claus reactions and the hydrolysis of carbonyl sulfide andcarbon disulfide are exothermic, resulting in an increase in temperaturein catalytic extension 24. Boiler catalytic effluent 120 is at a boilercatalytic effluent temperature. The boiler catalytic effluenttemperature can be between 250 deg C. and 400 deg C. Boiler catalyticeffluent 120 can contain hydrogen sulfide, sulfur-conversion products,process gases, process contaminants, and combinations of the same. Theexact composition of boiler catalytic effluent 120 depends on thecomposition of acid gas feed 100, the conditions in reaction furnace 10,including the minimum reaction temperature, and the catalyst incatalytic extension 24. The amount of hydrogen sulfide, process gases,process contaminants, and sulfur-containing contaminants present inboiler catalytic effluent 120 are reduced relative to the amount ofthose components present in reaction effluent 110.

Returning to FIG. 2, boiler catalytic effluent 120 exits extended boilerstage 20 and can be introduced to condenser 30.

Condenser 30 can reduce further the temperature of boiler catalyticeffluent 120 causing the elemental sulfur to condense as liquid sulfur134. The temperature of liquid sulfur 134 is between 120 deg C. and 155deg C. and alternately between about 125 deg C. and about 150 deg C.Liquid sulfur 134 contains greater than 95 wt % elemental sulfur,alternately greater than 97 wt % elemental sulfur, alternately greaterthan 99 wt % elemental sulfur, alternately greater than 99.5 wt %elemental sulfur, and alternately greater than 99.9 wt % elementalsulfur. The heat energy captured from boiler catalytic effluent 120 canbe used to heat condenser feed water 132 to produce low pressure steam135. Condenser feed water 132 can be any source of water suitable foruse to produce steam. In at least one embodiment, low pressure steam 135is a low pressure saturated steam at a pressure of 50 psig.

The other components present in boiler catalytic effluent 120, that donot condense as part of liquid sulfur 134, exit condenser 30 in gasesstream 130. Gases stream 130 can include sulfur-conversion products,hydrogen sulfide, process gases, process contaminants, andsulfur-containing contaminants. The exact composition of gases stream130 depends on the composition of boiler catalytic effluent 120. In atleast one embodiment, gases stream 130 contains less than about 1% byvolume elemental sulfur in the vapor phase.

Gases stream 130 can be introduced to thermal oxidizer 40. Fuel gasstream 142 and air stream 144 can be introduced to thermal oxidizer 40.Air stream 144 can be the same source or a different source as air feed104. Air stream 144 provides oxygen to thermal oxidizer 40 instoichiometric excess to the sulfur-containing contaminants present ingases stream 130. In at least one embodiment, the volumetric flow rateof air stream 144 results in an excess oxygen content of 2% by volume inthermal oxidizer 40. Fuel gas stream 142 can be the same source of fuelgas as fuel gas 102 or a different source of fuel gas. Thermal oxidizer40 can combust the hydrogen sulfide and sulfur-containing contaminantsin the presence of excess oxygen to create sulfur dioxide along withother combustion products to create oxidizer outlet stream 140. Oxidizeroutlet stream 140 can contain sulfur dioxide, process gases, traceamounts of sulfur-containing contaminants, and combinations of the same.Oxidizer outlet stream 140 has a reduced amount of sulfur-containingcontaminants relative to gases stream 130. In at least one embodiment,elemental sulfur present in gases stream 130 is converted to sulfurdioxide in thermal oxidizer 40. Thermal oxidizer 40 can operate at atemperature between 530 deg C. and 750 deg C., alternately between 538deg C. and 750 deg C., alternately between 550 deg C. and 750 deg C.,alternately between 575 deg C. and 725 deg C., and alternately between600 deg C. and 700 deg C. In at least one embodiment, thermal oxidizer40 is in the absence of catalyst.

Oxidizer outlet stream 140 can be introduced to scrubbing unit 50.Scrubbing unit 50 can be any type of scrubbing unit using a regenerablescrubbing medium capable of separating or removing an amount of sulfurdioxide from a process stream. Scrubbing unit 50 separates sulfurdioxide from oxidizer outlet stream 140 to produce recycle stream 150and effluent gases stream 155. In scrubbing unit 50, the regenerablescrubbing medium can capture the sulfur dioxide in a first step and thenrelease the sulfur dioxide in a second step. This allows scrubbing unit50 to operate in a loop.

Recycle stream 150 contains the amount of sulfur dioxide separated fromoxidizer outlet stream 140. In at least one embodiment, recycle stream150 contains sulfur dioxide saturated with water. In at least oneembodiment, the amount of sulfur dioxide in recycle stream 150 isgreater than about 90.0% by volume, alternately between 90.0% by volumeand 97.0% by volume, and alternately less than or equal to 98.0% byvolume. Recycle stream 150 is recycled to reaction furnace 10. In atleast one embodiment, the volumetric flowrate of air feed 104 can beadjusted based on the volumetric flow rate of recycle stream 150.

The gases remaining exit scrubbing unit 50 in effluent gases stream 155.Effluent gases stream 155 can contain process gases, sulfur dioxide,trace levels of contaminants, and combinations of the same. Effluentgases stream 155 can contain sulfur dioxide in an amount less than 2% byvolume, alternately less than 1% by volume, alternately less than 0.1%by volume, alternately less than 0.01% by volume, alternately less than0.001% by volume, alternately less than 0.0001% by volume, alternatelyless than 0.00005% by volume, alternately less than 0.00001% by volume.Effluent gases stream 155 can be sent for further processing, vented toatmosphere, or used in another processing unit. In at least oneembodiment, effluent gases stream 155 is vented to atmosphere.

Various process control elements can be included in the sulfur recoveryunit to provide for better control of the process units and the overallconversion of hydrogen sulfide and sulfur-containing contaminants toelemental sulfur.

With reference to FIG. 4, tail gas analyzer 35 can be installed aftercondenser 30 to analyze the composition in gases stream 130. Tail gasanalyzer 35 can be any type of analytical instrumentation capable ofmeasuring one or more components in a gas stream. In at least oneembodiment, tail gas analyzer 35 analyzes the amount of hydrogen sulfideand the amount of sulfur dioxide in gases stream 130. In at least oneembodiment, the composition of gases stream 130 as determined by tailgas analyzer 35 can be used to adjust the flow rate of air feed 104 aspart of a feedback control loop to maintain a stoichiometric ratio ofhydrogen sulfide to sulfur dioxide of 2:1. Temperature sensor 15 can beincluded in reaction furnace 10. Temperature sensor 15 can be any typeof instrument capable of measuring a temperature in the reactionfurnace. In at least one embodiment, temperature sensor 15 can be usedto adjust the flow rate of fuel gas 102 as needed to maintain or reachthe minimum reaction temperature. Advantageously, the ability to controlsulfur recovery unit 1 with instrumentation contributes to thesimplicity of sulfur recovery unit 1.

The overall conversion of hydrogen sulfide and sulfur-containingcontaminants to elemental sulfur in sulfur recovery unit can be greaterthan about 99 mole percent (mol %), alternately greater than about 99.2mol %, alternately greater than about 99.4 mol %, alternately greaterthan about 99.6 mol %, alternately greater than about 99.8 mol %,alternately greater than about 99.9 mol %, and alternately greater than99.99 mol %.

Referring to FIG. 5 and with reference to FIG. 3, an alternateembodiment of reaction furnace 10 and extended boiler stage 20 isprovided. Reaction furnace 10 can be physically connected to extendedboiler stage 20 such that reaction effluent 110 passes from reactionfurnace 10 to wasteheat stage 22 without entering external piping.

The sulfur recovery unit can be in the absence of a conventional Clauscatalytic stage. The sulfur recovery unit does not include a bauxitecatalyst, such that the catalyst section is in the absence of bauxite.The scrubbing unit can be in the absence of a caustic scrubbing mediumbecause caustic scrubber is non-regenerable and even small amounts ofelemental sulfur vapor could solidify in a caustic scrubber. Thescrubbing unit can be in the absence of oxygen as a scrubbing medium.

Although the embodiments have been described with respect to certainfeatures, it should be understood that the features and embodiments ofthe features can be combined with other features and embodiments of thefeatures.

Although the embodiments have been described in detail, it should beunderstood that various changes, substitutions, and alterations can bemade hereupon without departing from the principle and scope.Accordingly, the scope of the embodiments should be determined by thefollowing claims and their appropriate legal equivalents.

The singular forms “a,” “an,” and “the” include plural referents, unlessthe context clearly dictates otherwise.

Optional or optionally means that the subsequently described event orcircumstances can or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed as from about one particular value, and/or toabout another particular value. When such a range is expressed, it is tobe understood that another embodiment is from the one particular valueand/or to the other particular value, along with all combinations withinsaid range.

Throughout this application, where patents or publications arereferenced, the disclosures of these references in their entireties areintended to be incorporated by reference into this application, in orderto more fully describe the state of the art, except when thesereferences contradict the statements made here.

As used here and in the appended claims, the words “comprise,” “has,”and “include” and all grammatical variations thereof are each intendedto have an open, non-limiting meaning that does not exclude additionalelements or steps.

As used here, terms such as “first” and “second” are arbitrarilyassigned and are merely intended to differentiate between two or morecomponents of an apparatus. It is to be understood that the words“first” and “second” serve no other purpose and are not part of the nameor description of the component, nor do they necessarily define arelative location or position of the component. Furthermore, it is to beunderstood that that the mere use of the term “first” and “second” doesnot require that there be any “third” component, although thatpossibility is contemplated under the scope of the embodiments.

What is claimed is:
 1. A process for recovering sulfur in a sulfurrecovery unit, the process comprising the steps of: introducing an acidgas feed to a reaction furnace, where the acid gas feed compriseshydrogen sulfide; introducing a fuel gas to the reaction furnace;introducing air feed to the reaction furnace, where the air feedcomprises oxygen; reacting the hydrogen sulfide and oxygen in thereaction furnace at a minimum reaction temperature to produce a reactioneffluent, where the reaction effluent comprises elemental sulfur andsulfur dioxide, where the reaction effluent is at the minimum reactiontemperature; introducing the reaction effluent to a wasteheat stage ofan extended boiler stage; reducing the temperature of the reactioneffluent from the minimum reaction temperature to a boiler sectionoutlet temperature to produce a cooled effluent, where the cooledeffluent comprises hydrogen sulfide, sulfur dioxide, andsulfur-containing contaminants, where the temperature of the reactioneffluent is reduced by capturing heat energy; introducing the cooledeffluent to a catalytic extension of the extended boiler stage, wherethe catalytic extension comprises a catalyst, where the catalyticextension has a gross hourly space velocity between 3000 h−1 and 6000h−1; reacting the hydrogen sulfide, sulfur dioxide, andsulfur-containing contaminants in the catalytic extension to produce aboiler catalytic effluent, where the boiler catalytic effluent compriseselemental sulfur, where the boiler catalytic effluent is at a boilercatalytic effluent temperature; introducing the boiler catalyticeffluent to a condenser, where the boiler catalytic effluent compriseselemental sulfur; reducing the boiler catalytic effluent temperaturesuch that the elemental sulfur condenses to form liquid sulfur and agases stream; introducing the gases stream to a thermal oxidizer, wherethe gases stream comprises hydrogen sulfide and sulfur-containingcontaminants; introducing an air stream to the thermal oxidizer, wherethe air stream comprises oxygen; reacting the hydrogen sulfide andsulfur-containing contaminants with the oxygen to produce an oxidizeroutlet stream, where the oxidizer outlet stream comprises sulfurdioxide; introducing the oxidizer outlet stream to a scrubbing unit; andseparating the sulfur dioxide in the scrubbing unit to produce a recyclestream and an effluent gases, where the recycle stream comprises sulfurdioxide.
 2. The process of claim 1, further comprises the step ofintroducing the recycle stream to the reaction furnace.
 3. The processof claim 1, further comprises the steps of: introducing a boiler feedwater to the extended boiler stage; and increasing the temperature ofthe boiler feed water to produce a high pressure steam, where the heatcaptured from the reaction effluent is used to increase the temperatureof the boiler feed water.
 4. The process of claim 1, where the minimumreaction temperature is between 850 deg C. and 1250 deg C.
 5. Theprocess of claim 1, where the boiler section outlet temperature isbetween 148 deg C. and 254 deg C.
 6. The process of claim 1, where theboiler catalytic effluent temperature is between 250 deg C. and 400 degC.
 7. The process of claim 1, where the catalyst is titania extrudate.8. The process of claim 1, where an overall conversion of sulfurcompounds to elemental sulfur is greater than 99.9 mol %.
 9. The processof claim 1, further comprising the steps of: analyzing a composition ofthe gases stream in a tail gas analyzer; and adjusting a flow rate ofthe air feed based on the composition of the gases stream to maintain astoichiometric ratio of hydrogen sulfide to sulfur dioxide of 2:1. 10.The process of claim 1, further comprising the steps of: measuring theminimum reaction temperature in the reaction furnace with a temperaturesensor; and adjusting a flow rate of the fuel gas to maintain theminimum reaction temperature.